The invention relates to conveyor apparatus comprising a main conveyor by which mail items are routed by being nipped between a transport belt and a main belt, and a secondary conveyor towards which the mail items are routed, after switching, along a secondary belt, said main and said secondary belts performing the main conveying and the secondary conveying functions being constituted by a single, common belt that is twisted twice.
Such conveyor apparatus is known from Patent Document FR-2 555 974. It is used for switching mail items and stacking them in the sorting outlets of a postal sorting machine. In that known conveyor apparatus which is shown very diagrammatically in FIG. 1, a mail item 1′ is transported by a main conveyor 2′ and is switched by a switching flap 3′ towards a stacker conveyor 4′ of a sorting outlet stacker 5′ of a postal sorting machine. The main conveyor 2′ is constituted by a transport belt 6′ and by a main belt 7′ that extends between two sorting outlets and that runs along the belt 6′. The mail item is nipped between the two belts 6′ and 7′ so as to be routed towards a sorting outlet. The transport belt 6′ is driven by at least one motor-driven friction pulley P′tr. The stacker conveyor 4′ is constituted by a stacker belt 8′ which extends in the bottom of the stacker 5′. As can be seen in FIG. 1, the main belt 7′ and the stacker belt 8′ are formed by a single, common endless belt that is twisted twice and that is designated by overall reference 9′ in FIG. 1. That twice-twisted belt 9′ travels over a generally figure-of-eight shaped path so as to produce two movements for driving the mail items 1′, which movements are in the same direction, are substantially parallel, and are of equal speeds. In the example shown in FIG. 1, the belt 9′ is engaged over four main pulleys P′ that define the figure-of-eight shaped path, and it has rectilinear segments 10′, 11′ that are substantially superposed, and that cross over at the intersection between the two loops of the figure-of-eight shaped path. The pulleys P′ can be idler pulleys, while the single motor-driven pulley drives the transport belt 6′ and also the twice-twisted belt 9′ which is in friction contact with the belt 6′. That arrangement guarantees, in simple manner, that the two belts 6′ and 9′ are driven at an identical drive speed.
In such conveyor apparatus, it has been observed that the path of the belt 9′ can vary in the vertical plane, which is detrimental to proper operation of the stackers of the sorting machine. Such untimely displacements of the belt 9′ are presently eliminated by regularly and manually re-centering the belt 9‘on the pulleys P’, which gives rise to non-negligible maintenance costs.